API that has KDE replacements

Qt classes that have their KDE versions

KDE applications should not use Qt classes that have their KDE replacements. Examples include QFileDialog, QDialog, QMainWindow. KDE versions of these classes provide better integration with KDE.

QSystemTrayIcon::showMessage()

Use KNotification for better KDE integration.

API that is considered broken

QWidget::showFullScreen/Maximized/Minimized/Normal()

Try to avoid calls to QWidget::showFullScreen(), QWidget::showMaximized(), QWidget::showMinimized() and QWidget::showNormal(), use them only if you understand what they do and you really want that. These calls do not only change the relevant window state as most people expect, but they also have other effects. For example, showFullScreen() among other things also removes the maximized state (see bug #157941), the same way showNormal() is not an inverse call to showFullScreen().

Either use QWidget::setWindowState() in the following way
widget->setWindowState(widget->windowState() | Qt::WindowFullScreen); // set
widget->setWindowState(widget->windowState() & ~Qt::WindowFullScreen); // reset
or use KDE API - KToggleFullScreenAction has setFullScreen() helper function, and KWindowSystem has calls for changing window state.